This invention relates to fused materials produced in a finely divided form. It relates specifically to a method making such finely divided materials particularly where these are ceramic in nature.
It is well known in the art that a material produced in a molten state is most preferably reduced to a more manageable temperature by a rapid quenching operation. This may be done by pouring the melt into water but this generates very large quantities of steam and requires a very large excess of water. In addition the water can be the source of impurities or undesirable reactions. To counter this problem it has been proposed to drop the fused material on to cooling media such as steel balls or into cavities between thick steel plates. For material dropped on to steel balls there is the continuing problem of separation of product from the cooling media. When material is dropped between steel plates, the plates deteriorate and need to be replaced at frequent intervals. Air or water quenching solves both these problems but does have the limitation of not providing crude material that can be crushed and sized to provide abrasive grit sizes of the full range required by the abrasives industry.
For ceramic and refractory materials however the preferred method is to air quench the molten materials. In this process the molten stream is injected into a high speed stream of cold air. The turbulence divides the stream into droplets which are then chilled by the air such that initially a shell forms around the droplet with the thickness increasing until all the droplet has solidified. This process too is not without its problems however since it is estimated that a droplet/particle needs to travel as much as 15 feet before the shell is strong enough to withstand inter-particle contacts that could rupture the shell and cause fusion between colliding droplet/particles. In addition the process generates a lot of dust that poses a collection and pollution problem.
A novel method has now been devised in which a molten ceramic material can be rapidly solidified with minimum generation of dust to obtain relatively uniform spherical particles. This method can be adapted to the production of a wide variety of ceramics including zirconia, yttria-stabilized zirconia, alumina, alumina/zirconia, magnesia and the like.